This invention relates generally to drums for debarking logs for pulp-making and more particularly to a log discharge control system for debarking drums.
Pulp-making for the paper industry requires removal of bark from the pulp wood logs since bark is not suitable for pulping operations. An established method for debarking is to tumble logs in a large horizontal rotating debarking drum in which the bark is removed by the impact and friction between the logs during tumbling.
Originally, debarking drums were provided with fixed gates as seen in FIG. 1 which established a given level of logs necessary for discharge from the debarking drum. This determined the length of tumbling time, or the retention time, in the drums.
Since bark-to-wood adhesion characteristics vary widely among the several pulp wood types processed, it is desirable to have capability for varying the retention time and degree of filling of logs in the debarking drum in order to provide the necessary impact and friction conditions to separate the bark from the wood. This capability may be provided by means of movable discharge gates to vary the size of the discharge opening--horizontal sliding gates as seen in FIG. 2, vertical sliding gates as seen in FIG. 3, or rotary discharge opening positioners as seen in FIG. 4. Horizontal and vertical sliding gates require large frames and large traversing distances in order to close the drum. Moreover, they often provide pinch points which interfere with smooth discharge of the logs. Rotary discharge opening positioners, which position an opening of fixed size, have permitted a more compact installation and are shaped to eliminate pinch points. They are generally preferable for this reason. However, rotary discharge opening positioners do endure log pounding against their entire solid surfaces. The resulting axial thrust and pounding can cause cocking of the positioner and may permit small logs to jam between the rotating drum and the stationary face of the rotary opening positioner.
Bark fragments and grit removed from the logs during tumbling also travel along the drum toward the discharge end. In order to prevent discharge of the bark and grit with the pulp logs, small slots or other openings are provided in the drum shell wall to discharge bark fragments and grit. Provision is also made at the discharge end of the debarking drum to deflect bark back into the drum for discharge through the slots while permitting discharge of the logs. This bark deflection is provided by either the gate or an annular dam installed in the drum at the discharge end as shown in FIGS. 5A, B, and C.
It has become the practice to provide pulp logs to the debarking drums in increasing lengths which frequently exceed the diameter of the drum and which present a potential jamming problem due to obstructions in the drum. The bark deflection dam of the drum, the discharge gate, or gate pinch points may provide such obstructions. Clearance of the resulting jams requires the largest possible opening. Existing rotary opening positioners only provide openings of about 25% of the area of the drum discharge area and may have to be removed in cases of severe jamming. Periodic jams or discharge of bark and grit along with the pulp logs are possible as long as these features are present in the debarking drum discharge system.
The foregoing illustrates limitations known to exist in present pulp wood debarking drum discharge systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.